How is the idle (is it very low or does it flutter) is there a check engine light on? I had a issue with a Nissan 3.0 not too long ago that had a similar problem (very low idle and when the ac was on it would stall) it had a code for the idle air control valve. If your car has the check engine light on try to have the computer scanned to see what the codes are.
Hope that helps some 🙂